The conventional printed circuit board is made of glass fiber reinforced resin, and is laser-drilled through its upper and lower surfaces to make through holes. A layer of metal is filled in the drilled through holes by electroplating to attain to vertical electrical connection. The electroplated metal layer covers the entire surface of the printed circuit board including the drilled through holes, therefore it cannot be provided for anisotropic electrical connection unless the metal layer connecting high-pitch via went through a highly accurate etching processes.
Conventionally Anisotropic Conductive Film (ACF) is an adhesive film with anisotropic conductivity, which is used in the connection and adherence of electrical devices, such as the anisotropic conductive film disclosed in U.S. Pat. No. 6,344,156 entitled “ANISOTROPIC CONDUCTIVE ADHESIVE FILM”. The anisotropic conductive adhesive film is used in outer lead bonding (OLB), bump connection of the Tape Carrier Package (TCP) or combination of IC chips with the LCD panels by Chip-On-Glass bonding. The anisotropic conductive film is an insulating resin containing conductive particles properly. To acquire vertical conductivity, the conductive particles must have same size and with uniformity distribution. Meanwhile, the electrical devices should have extruded electrodes, such as bumps or pillar, to stick into the anisotropic conductive film. When the distance between the extruded electrode and outer circuit board is close enough corresponding to the diameter of an electric particle, the anisotropic conductivity can be acquired. Therefore, the conventional anisotropic conductive film is used in the adherence of electrical devices and outer circuit board, but it is improper to an interposer between semiconductor burn-in testing apparatus and wafer under test due to adhesion and reactivity of the anisotropic conductive film.